The production of polymers and copolymers of lower .alpha.-olefins, particularly ethylene and propylene, has gained substantial commercial significance. The polymeric products are relatively inexpensive and exhibit a number of commercially useful properties. In the case of the polymerization of ethylene, the process is relatively uncomplicated in that the product type is not influenced by the manner in which the ethylene molecules add to the growing polymeric chain and the product does not exist in stereoisomeric forms.
In the case of the polymerization of propylene, however, the presence of pendant methyl groups on the polymeric chain results in the possibility of several product types depending upon the stereoregularity with which the propylene molecules add to the growing chain. Much, if not most, of the commercial polypropylene is crystalline and results from the stereoregular addition of propylene molecules in a regular head-to-tail manner. Polymer resulting from the addition of propylene units in a random and irregular manner is termed atactic. This amorphous form is less desirable and, if atactic polymer is present in substantial quantities, must be removed as by an extraction step in order to provide the more desirable crystalline polymer.
Also significant from a commercial standpoint is the activity of the polymerization catalyst. A number of the early polymerization catalysts, e.g. trivalent titanium, chromium or vanadium catalysts, were of a relatively low activity and the polymeric product contained significant proportions of catalyst residues. The removal of such residues as by a deashing step was required in order to obtain commercially acceptable properties.
The more recent olefin polymerization catalysts are more stereo-regulating and of sufficient catalyst activity so that extraction and/or deashing steps are not required. In the terms now employed conventionally to describe such catalysts, the high activity olefin polymerization catalysts are formed from a procatalyst which typically contains magnesium, titanium and halogen moieties, a cocatalyst which is typically an organoaluminum compound and a selectivity control agent which may be provided as a partial or a total complex with the cocatalyst. Although each of these components has a significant influence on the polymerization process and the polymer product produced thereby, the nature of the catalyst as well as the polymer product seems to be most influenced by the particular nature of the procatalyst. Much of the research directed toward improvement of the olefin polymerization catalyst has been devoted to improvement of the procatalyst.
Kioka et al, U.S. Pat. No. 4,330,649, describe a solid catalyst component (procatalyst) obtained by heating a soluble magnesium compound such as magnesium chloride with a higher alcohol in the presence of an ester to produce a solution. This solution is added to titanium tetrachloride and an electron donor to form the procatalyst. Band, U.S. Pat. No. 4,472,521, reacts a magnesium alkoxide with excess titanium alkoxide, wherein each alkoxide has 4 or more carbons in the presence of aromatic hydrocarbon. Titanium tetrachloride and an electron donor are added to the resulting solution to produce a solid procatalyst which is post-treated with transition metal halide.
A number of the more attractive olefin polymerization procatalysts are produced from magnesium alkoxides wherein the alkoxide moieties have one or two carbon atoms. Magnesium ethoxide appears to be a particularly attractive procatalyst precursor. The use of magnesium ethoxide poses a somewhat unique problem in that, unlike other magnesium alkoxides, magnesium ethoxide is not readily soluble in the corresponding alkanol, i.e., ethanol. Various measures have been proposed for the solubilization of magnesium alkoxide including the formation of complex magnesium ethoxides as disclosed by Job, U.S. Pat. No. 4,710,428.
Several procedures have been disclosed which involve the solubilization of magnesium ethoxide by reaction with carbon dioxide in ethanol. Arzoumanidis, U.S. Pat. No. 4,540,679, produces an olefin polymerization catalyst component by contacting a suspension of magnesium ethoxide in ethanol with carbon dioxide. To the resulting solution is added an organoaluminum compound in hydrocarbon solution to produce granular particles which are employed as a support for the titanium species which result from contacting the granular particle with titanium tetrachloride. Nestlerode et al, U.S. Pat. No. 4,728,705, react magnesium ethoxide in ethanol with carbon dioxide to form a solution. This solution is spray dried to produce particles or alternatively is used to impregnate carrier particles. The particles resulting from either modification are useful in the production of an olefin polymerization procatalyst of particularly desirable morphology.
The reaction of magnesium ethoxide with carbon dioxide in ethanol produces a soluble complex containing moieties of magnesium, ethoxide and carbon monoxide which is often referred to as carbonated magnesium ethoxide or CMEO. The precise structure of the complex is somewhat uncertain but a low pressure stable form is believed to be illustrated by the formula EQU Mg.sub.2 (OEt).sub.4 (CO.sub.2).sub.3 (I)
and the complex is soluble in ethanol. Although the above references teach methods of converting this carbon dioxide-containing complex to olefin polymerization procatalysts and thence to olefin polymerization catalysts, it would be of advantage to provide improved olefin polymerization procatalysts and catalysts from the carbonated magnesium ethoxide complex.